The present invention relates generally to linear stepper motors and more particularly to systems and methods for precisely positioning an object using a linear stepper motor.
Linear stepper motors typically include a platen 10 and a forcer 20 as shown, for example, in FIG. 1. The platen 10 is the fixed, passive part of the motor and its length will determine the distance the motor will travel. The platen 10 typically has a number of teeth 12 equally spaced along its length, and is like the rotor in a traditional rotary stepper motor, except it is not a permanent magnet. The forcer 20 typically includes four pole pieces 22 that each have three or more teeth 24. The pitch of each tooth 24 is staggered with respect to the teeth 12 of the platen 10. The forcer 20 uses mechanical roller bearings or air bearings to ride above the platen on an air gap so that the two never physically come into contact with each other. The magnetic field in the forcer is changed by passing current through its coils (e.g., coils A and B). This action causes the next set of teeth 24 to align with the teeth 12 on the platen and causes the forcer to move from tooth to tooth over the platen in stepwise travel. When the current pattern is reversed, the forcer reverses its direction of travel. A linear stepper motor controller controls the current energizing pattern (switching cycle) applied to the coils so that the motor moves in a stepwise, controllable manner. A complete switching cycle includes four full steps, which moves the forcer 20 the distance of one tooth pitch over the platen 10. In a microstepping mode, the forcer can be controlled to move a fraction of a full step, for example, by breaking each applied current step into multiple current steps. For example, multiple current steps can be applied between full positive, zero current and full negative. The typical resolution of a linear motor is of the order of 100 full steps per inch. In microstepping mode, the resolution is greatly increased.
It is known that linear stepper motors are limited in terms of precision and smoothness of motion due, for example, to the presence of cyclic positioning error, thermal expansion error, straightness of the actuator, and other errors due to imperfection in manufacturing of the motor. As a consequence, an error may be introduced between the desired and actual position of the object moved by the motor, especially if the object is located at some distance from the center of the forcer. Also, an error may be introduced between the desired and actual orientation of an object attached to the moving part of the motor (i.e., forcer 20). This is illustrated in FIG. 2.
The positioning of the forcer (point C in FIG. 2) is affected by the cyclic, thermal and random errors in the platen teeth. However, the position of the object P that is off-centered from the ideal line of the motion (X) is affected not only by the positioning error in point C, but also the orientation error (φ), which is mostly due to imperfect straightness of the platen. The positioning error of the object P along the axis X′ that is parallel to the axis X can be described as E=Ex+r sin (φ), where Ex is the positioning error in point C, r is the distance between points C and P, and φ is the orientation error. Clearly, the positioning error E is critical in achieving high precision positioning of the object P. It is therefore desirable to provide systems and methods to compensate for not only the linear position error Ex of the forcer; but also the total error E.
For some applications, such as a microarraying apparatus, or a high-precision X-Y stage used in various instruments and machines, it is highly desirable to achieve precise and smooth motion within given specifications. In many applications the solution is based on a lead-screw mechanism rotated by a DC, AC, or stepper motor, and a linear guide that guides the object along the lead-screw. However, the construction of a lead-screw mechanism is not suitable for high speed, high-acceleration applications. For such applications, a mechanism that is based on a linear motor and linear optical encoder offers a range of advantages over the traditional lead-screw mechanism: no transmission (gear), very small friction (when wheels are used) or negligible (when air-bearings are used), very high speed (often exceeding several meters per second), very high accelerations (often exceeding 1G), flexibility in length (platen can be made several meters long), etc. Typically, the linear encoder or some other measuring device is used as a feedback device. However, the cost of typical brushless DC or AC linear motor and a linear optical encoder may be prohibitively expensive and would qualify this technology for “high-end” applications. In other words, many laboratory instruments and machines cannot afford this technology from a cost of goods standpoint.
Hysteresis is an effect that exists in most of electromechanical systems. Linear stepper motors also show differences between achieved positions when moving to the same desired position from two different directions. In some applications, such as printing of biological samples in a form of an array, it may be very important to achieve high positioning precision, regardless of the direction of motion towards the desired position.
Control of linear stepper motors in microstepping mode is performed using microstepping stepper motor drivers. Although these drivers are calibrated to achieve equal distance between microsteps, there exist differences in actual positions of the object attached to the forcer, if different drivers are used with the same motor. The impact of driver characteristics is also one of the effects that should be taken into account when high positioning precision is desired.
Therefore, it would be desirable to provide systems and methods that combine a relatively affordable linear stepper motor with an advanced controller that would provide high positioning precision, but without any position-measuring device. Further, such systems and methods should provide enhanced movement and positioning precision capabilities, including the ability to compensate for cyclic error and hysteresis.